1. Field of the Invention
The present invention relates to a rotary machine used in water containing large quantity of soil and sand, such as water turbine for use in a hydraulic power station, as well as to a moving blade of such a water turbine. More particularly, the present invention is concerned with a water turbine and a moving blade for use on such a water turbine which are improved to exhibit greater resistance to erosion caused by cavitation at the portions which contact the flowing water, as well as greater resistance to wear caused by soil and sand contained in the water.
2. Description of the Related Art
In general, water-contactable portions of a water turbine for use in a hydraulic power station, in particularly the runner and the guide vanes, tend to suffer from erosion caused by a cavitation which takes place between the material of such water-contactable portions and water. Erosion is also caused by wear of the material due to attack of solid contents, e.g., Al.sub.2 O.sub.3, SiO.sub.2, Fe.sub.2 O, MgO and CaO, as well as clay. In general, power generating water turbines have been used mainly in the regions where water contains comparatively small quantity of sand and soil. Hitherto, therefore, no effort has been done for the purpose of developing materials of water-contactable parts of water turbines which have high resistance to wear caused by soil and sand. In other words, developments of water-contactable parts have been concentrated to materials which have high resistance to erosion caused by cavitation. Erosion of moving part such as moving blades due to cavitation causes a large vibration and noise during running of the water turbine, and seriously reduces the efficiency of the water turbine. In order to suppress erosion due to cavitation, Ni-containing 13Cr cast steel subjected to quench-hardening and tempering to develop required strength has been mainly used as the material of parts of runners of water turbines. The Ni-containing 13Cr cast steel exhibits superior resistance to erosion caused by cavitation as compared with ordinary Cr cast steel but is still unsatisfactory. Under these circumstances, it has been a common measure to pad the portion of a runner where cavitation takes place with a metal having high resistance to cavitation containing ferrite precipitated in austenitic matrix. An example of such cavitation-resistant metal is JIS D 308 containing not more than 0.08 wt% of C, nor more than 0.9 wt% of Si, not more than 2.5 wt% of Mn, 9 to 11 wt% of Ni, and 18 to 21 wt% of Cr. Another Example is JIS D 309Mo which contains not more than 2.5 wt% of Mn, 12 to 14 wt% of Ni, 22 to 25 wt% of Cr and 2 to 3 wt% of Mo.
The padding with JIS D 308 or JIS 309 Mo metal provides an appreciable effect in preventing erosion by cavitation, but is ineffective in suppressing wear by soil and sand contained in water. As to the above-described related art, reference shall be made to Japanese Patent Examined Publication Nos. 61-11312, 61-11311 and 62-56947.
The padding with JIS D 308 or JIS D 309 Mo is intended mainly for prevention of erosion of runners of water turbines attributable to cavitation, not to mention improvement in corrosion resistance. Padding with such metals, however, cannot provide satisfactory result in regard to prevention of erosion caused by wear due to attack of soil and sand. This is attributable to the fact that the mechanism of the erosion by wear caused by soil and sand is entirely different from that of erosion caused by cavitation. The erosion by cavitation is a phenomenon in which an erosion of a material is caused by a large impacting force which is generated when a cavity formed on a member moving at high speed in water is collapsed. On the other hand, erosion by wear is a phenomenon in which a water-contactable material is mechanically cut and ground by solid particles such as soil and sand particles contained in water. Obviously, the degree of wear due to soil and sand depends on flow velocity. In general, the greater the collision impact, the heavier the erosion. Conventional padding materials such as JIS D 308 and JIS D 309 Mo is work-hardened by the high impact pressure generated at the time of collapse of cavity so as to provide a hard surface on the water-contactable portion padded with such metal. Unfortunately, this material could not provide sufficient effect in prevention of erosion, due to the fact that erosion actually takes place in the form of a combination of erosion by cavitation and erosion caused by wear due to attack of soil and sand. Hitherto, no material has been developed which has high resistance both to erosion by cavitation and erosion to wear.